1. Technical Field
This invention relates to the field of clutches for selectively coupling a deriving member to a driven member. More particularly, the invention pertains to such a clutch having soft start characteristics for use with a vehicle air conditioning compressor.
2. Description of the Prior Art
A compressor for an air conditioning system of an automobile typically includes a clutch to enable disengagement of the compressor during periods in which no air conditioning is desired. During demand situations, the clutch is engaged so that the compressor compresses the refrigerant in a known manner. Prior art clutches typically are of an electromagnetic friction clutch construction. A compressor typically is driven by a V-groove pulley assembly supported by a bearing pressed onto the body of the compressor. The pulley is driven by the automotive accessory drive belt.
Prior art clutches typically include an armature affixed to the splined shaft of the compressor via a bolt. A stationary wire coil assembly is press-fit to the compressor housing. The armature is normally separated by a small air gap from the pulley face by a spring mechanism, so that the clutch is normally disengaged. Automotive voltage, normally having a nominal value of 14.4 volts, is applied to the coil to engage the clutch. Upon application of this voltage, the current in the coil increases from zero on a time scale related to the inductive time constant of the clutch, typically 150 milliseconds. The current induces magnetic flux to flow in the pulley, across the air gap, and into the armature. When the magnetic flux density reaches a critical level, the attractive force between the armature and the pulley becomes large enough to overcome the spring force holding the armature away from the pulley. The armature is then rapidly drawn into contact with the pulley, suddenly initiating torque transfer to the compressor and causing the compressor shaft to begin to rotate. When the shaft speed matches the pulley speed, the torque then reaches a steady-state level that is a function of pulley speed, cooling demand, and other vehicle operational characteristics.
When such a prior art clutch is engaged, undesirable effects can occur, including stumble, surge, and noise generation. Stumble is a longitudinal vehicle vibration induced by the sudden change in engine torque demand upon compressor engagement and surge is similar vibration upon disengagement. Noise is generated as the armature of the electromagnetic friction clutch is rapidly driven into contact with the clutch pulley during engagement. These effects of rapid compressor engagement are objectionalbe to the vehicle occupants and may contribute to premature failure of compressor components. Previous efforts to overcome these concerns include using passive mechanical means, such as mating slots or other structures provided in the pulley and armature, which are said to reduce the rate of increase of magnetic force. An example of this is illustrated in U.S. Pat. No. 4,749,073 to Olden.
Another attempt to reduce these concerns includes the insertion of an elastomeric coupling between the armature and compressor shaft to damp transients encountered during engagement and operation, as shown in U.S. Pat. No. 5,219,273 to Chang. Other attempts to reduce these concerns include electronic controls of the clutch current in an attempt to produce soft-start coupling, as described in U.S. Pat. No. 4,509,091 to Booth and U.S. Pat. No. 4,567,975 to Roll. These patents disclose a method of generating a time-varying clutch current. These methods draw the armature initially to the pulley, but allow the pulley to slip. The clutch current is increased smoothly to gradually increase the level of torque transfer and decrease the slip until a state of complete engagement is reached. These methods permit slip, which causes the electromagnetic friction clutch surfaces to become burnished over time, reducing the ability of the clutch to transfer torque.
It would therefore be desirable to provide a clutch which softens the engagement of an air conditioning compressor, but which does not cause excessive wear of the clutch friction surfaces.
Accordingly, an object of the present invention is to provide, in an air conditioning compressor a clutch having a soft start which does not experience excessive wear.
An advantage of the use of a clutch according to the present invention is that the air conditioning system will have less objectionable noise and vibration. A second advantage is that, by reducing the objectionable characteristics of such a system, one may cycle the clutch more frequently and thereby maximize fuel efficiency and optimally control the temperature of the passenger compartment.
Further advantages include having the ability to operate the compressor at high speeds. With conventional clutches, operation at high engine RPMs causes undesirable noise. With a clutch according to the present invention, the clutch may be slipped, permitting lower RPM operation for a given input speed.
The present invention concerns an electromagnetic clutch for selectively coupling a rotating driving member with a driven member to be rotated. The clutch includes a generally cylindrical driving member having an axis of rotation and an annular cavity formed therein with a cavity friction surface and a driven member having an axis of rotation coaxial with the driving member axis of rotation, the driven member having an axially extending leg positioned in the cavity with a leg friction surface positioned radially adjacent the cavity friction surface to form an annular space therebetween. A quantity of flowable magnetic material provided in the annular space and a magnetic coil is positioned adjacent the annular space and spaced axially from the cavity. When the driving member and the driven member are mounted for rotation such that the driving member can be rotated without rotating the driven member, and when electrical power is supplied to the magnetic coil, magnetic flux is created in the annular space polarizing the magnetic material and frictionally coupling the friction surfaces to cause the driven member to rotate with the driving member.